A light emitting display is a display device which uses a plurality of light emitting elements to display an image. Each of the light emitting elements emits light according to an applied current. Particularly, an organic light emitting diode display uses an organic light emitting cell as the light emitting element, and the organic light emitting cell has characteristics of a diode and can be referred to as an organic light emitting diode (OLED). The organic light emitting cell includes an anode, an organic thin film, and a cathode.
According to an addressing method, methods for driving the organic light emitting cells may be classified into a passive matrix method or an active matrix method. In the passive matrix method, the organic light emitting cells are formed between anode lines and cathode lines perpendicularly crossing the anode line, and driven by selecting the respective lines. In the active matrix method, a thin film transistor is coupled to each pixel electrode (e.g., an anode line), and the organic light emitting cells are driven according to a voltage maintained by a capacitor coupled to a gate of a thin film transistor. Further, depending on formats of signals applied to the capacitor for maintaining the voltage, the active matrix method may be categorized as either a voltage programming method or a current programming method.
A pixel circuit according to the voltage programming method has difficulties in obtaining high gray scales because of deviations in threshold voltages and/or in electron mobilities of thin film transistors, the deviations being caused by non-uniformity of a manufacturing process. On the other hand, according to the current programming method, uniform display characteristics are achieved even though driving transistors in each pixel have non-uniform voltage-current characteristics, provided that a current source for supplying the current to the pixel is uniform throughout the whole panel (i.e., all the data lines).
However, in the light emitting display using the current programming method, it is necessary to provide a data driver which converts a data signal representing a gray scale to an analog current (hereinafter, “data current”) to be applied to a data line coupled to the pixel circuit.
The data driver needs a digital/analog converter for converting the digital data signal to the analog data current and an output stage for buffering and outputting the converted data current. Generally, before the data currents are transmitted to the data lines during one horizontal period, the output stage has to buffer the data currents corresponding to the pixel circuits on one row during the horizontal period. However, as the resolution of a light emitting display becomes higher, a horizontal period becomes shorter. Because of this, the output stage may not be able to buffer the data currents during the horizontal period when a magnitude of a data current is small. As a result, the data currents can be improperly transmitted to the data lines.